Large seasonal variation in phytoplankton production in the Amundsen Sea

Abstract

To better estimate annual primary production in the Amundsen Sea, which is one of the highest productivity regions in the Southern Ocean, the seasonal variations in carbon and nitrogen uptake rates of phytoplankton were investigated in this study. Based on 13 C-15 N dual isotope tracer techniques, the average daily productivities for the Amundsen polynya (AP), Pine Island polynya (PIP) and nonpolynya regions were 0.25, 0.16 and 0.12 g C m -2 day -1 , respectively. The average daily uptake rates of total nitrogen were 60.2, 53.5 and 34.8 mg N m -2 day -1 for the AP, PIP and non-polynya stations, respectively. In spite of the high concentration of nitrate in the Amundsen Sea, daily nitrate uptake rates (mean ± SD = 0.02 ± 0.01 g N m -2 day -1 ) were lower than ammonium uptakes for all productivity stations in this study, which resulted in a significantly lower f-ratio (mean ± SD = 0.44 ± 0.24) than that (mean ± SD = 0.71 ± 0.15) of the previous year. The substantially lower uptake rates of carbon and nitrogen and the f-ratio, especially in the AP, are due to a large seasonal variation in the uptake rates mainly caused by the shorter daytime duration and partly due to lower light availability induced by deeper mixed conditions in the present study compared with the previous study in 2010/2011. The large seasonal variation in daily phytoplankton production should be considered to better estimate annual production as a basic food source for higher trophic levels in the Amundsen Sea.

Small phytoplankton contribution to the standing stocks and the total primary production in the Amundsen Sea

“…Our rate (0.42 g C m −2 d −1 ) in the non-polynya region is somewhat higher than those reported previously but they are not significantly different (t test, p>0.05). In comparison, our total daily carbon uptake rate in the polynya region (0.84 g C m −2 d −1 ) is lower than that (2.2 g C m −2 d −1 ) of Lee et al (2012) and higher than that (0.2 g C m −2 d −1 ) of Kim et al (2015). The carbon uptake rates of phytoplankton in Lee et al (2012) and Kim et al (2015) were measured during the periods 21 December 2010 to 23 January 2011 and 11 February to 14 March 2012, respectively.…”

“…Our measurements in this study were executed mainly during the period 1-15 January 2014. For the Amundsen polynya region, a large seasonal variation in the total daily carbon uptake rate of phytoplankton was already reported by Kim et al (2015) and Arrigo et al (2012) based on field-measured data and satellitederived approach, respectively. Generally, late December is the time of peak uptake rate in this region .…”

“…According to the previous reports in the Amundsen Sea (Lee et al, 2012;Kim et al, 2015), the total daily carbon uptake rates ranged from 0.2 to 0.12 g C m −2 d −1 in the non-polynya region. Our rate (0.42 g C m −2 d −1 ) in the non-polynya region is somewhat higher than those reported previously but they are not significantly different (t test, p>0.05).…”

“…After the filters were extracted using the method described by Kim et al (2015), all chlorophyll a concentrations were subsequently determined onboard using a Trilogy fluorometer (Turner Designs, USA). The methods and calculations for chlorophyll a were based on Parsons et al (1984).…”

“…Using a dual stable isotope technique (Lee et al, 2012;Kim et al, 2015), the experiments of carbon and nitrogen uptake rates of phytoplankton were conducted at 12 selected productivity stations including two revisited stations (St. 3-1 and St. 19-1) when on-deck incubations were available during daytime at oceanographic survey stations. Water samples from the six light depths for the uptake experiments were obtained from a CTD-rosette sampler system equipped with twenty-four 10 L Niskin bottles.…”